Asymptomatic #COVID19 has seen a lot of disagreement this week: is it 15% or 40%? Is the transmission from asymptomatic people "very rare"? Some comments in this thread. @sophiescott2 @MelissaLDavey
1/ Important to separate (a) pre-symptomatic (= every infected person before symptoms!) from (b) asymptomatic throughout infection (so need follow up to check if symptoms develop - many studies don't) - which some commentary & reviews don't do. 
2/ There are 9 reasonably good studies of asymptomatic rates (checked everyone in a well-defined at-risk group & > 7 days follow up) when combined suggest a rate of around 15% medrxiv.org/content/10.110…
3/ Transmission rates also appear to be about 40% lower for asymptomatic vs symptomatic cases (4 of 9 studies assessed this). Why? Could be less time infectious or that not coughing – less droplets, less environment contamination.
4/ Combined this means < 10% of transmissions likely to be from asymptomatic cases - so uncommon though perhaps not quite "very rare" @mvankerkhove - but small enough that #TestTraceIsolate can pick up most.
5/ Transmission from pre-symptomatic cases likely more common, but good #TestTraceIsolate finds (and isolates) many of the pre-symptomatic & asymptomatic folk - data from Iceland shows 50% folk diagnosed while in quarantine after close contact with a known COVID19 case.
